This algorithm also takes into account if Excel gets upgraded again to handle more than 16k columns. If you really wanted to go overboard, you could pass in an additional value and replace the instances of 26 with another number to accomodate alternate alphabets

@KodeSeeker Let me see if I can explain that simply... Each digit has 26 symbols, but this isn't base 26. It's sort of like base 27, but without 0. n-- is a way to shift out the 0 from base 27, and we're left with the [1, 26] range mapping to [A, Z]. Makes sense?
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Samuel AudetOct 14 '13 at 11:36

If you don't want to use VBA, you can use this
replace colnr with the number you want

=MID(ADDRESS(1,colnr),2,LEN(ADDRESS(1,colnr))-3)

Please be aware of the fact that this formula is volatile because of the usage of the ADDRESS function. Volatile functions are functions that are recalculated by excel after EVERY change.
Normally excel recalculates formula's only when their dependent references changes.

There is one thing to be aware of. Starting with Office 2007, Excel actually has up to 16,384 columns. That translates to XFD (the old max of 256 colums was IV). You will have to modify these methods somewhat to make them work for three characters.